Surface-active, chemically resistant compounds are added to acid mordants and galvanic electrolyte solutions and to comparable acid baths for metal surface treatments because of the gas evolved in the process, especially hydrogen, and to a less extent also the anodic evolution of oxygen and the resulting spray mists of acid, corrosive and in some cases toxic aerosols.
Perfluoralkylated sulphonates having cations of the type of tetraalkylammonium, for example, are used for drastically reduced surface tension.
The desired high interface activity of the surface-active additive, however, is frequently accompanied by the formation of foam which under certain operating conditions and processes may in extreme cases prevent the normal development of the process and therefore makes the use of surfactants questionable.
This applies particularly to mordanting or galvanic baths which have a geometrically unfavorable configuration with great depth and, by comparison, small bath surface, or when the current densities are used which give rise to a correspondingly intensive evolution of hydrogen.
The formation of foam and its intensity depend on the surface tension, i.e. the quantity of surfactant, the temperature and the mechanical movement of the solution and mechanical destruction of the foam and the extent to which the bath is filled with workpieces to be treated at any one time.
It is already known to counteract this disturbing formation of foam by adding foam inhibiting agents, also known as defoamers, foam preventing agents or anti-foamants, such as fats, oils, long chained alcohols or glycols, alkylphosphates, metal soaps, special silicone defoamers, commercial perfluoroalkyl-modified hydrocarbon defoamers and perfluoroalkyl-substituted silicones to the mordants and baths (Kirk-Othmer (3.), Vol. 7: 430-48; Glodmann, Chem. Industrie 20 (1968), 335-7).
Fully fluorinated alkylphosphonates are described in German No. 2,233,941 as antifoamants for alkylsulphonates used in textile finishing. In corrosive media such as highly acid and oxidizing galvanic baths, these compounds are ineffective and undergo decomposition.
Conventional commercially available additives for foam prevention in surface-active solutions cannot be used in acid mordants and galvanic baths owing to the physical and chemical conditions prevailing there and the temperature, the flow of current, the low pH and the powerful oxidizing effect.
Commercial antifoamants cannot withstand the operating conditions (e.g. of pH values below 1), elevated temperatures, current densities possibly of about 80 A/dm.sup.2 and CrO.sub.3 concentrations (e.g. of 250 g/l) and therefore have no suitable antifoaming action under these conditions. Moreover, the products of decomposition of these agents are liable to contaminate the bath and may interfere with subsequent steps of the process of metal deposition or mordanting.
Special perfluoroalkyl-substituted phosphoric acid esters [R.sub.F SO.sub.2 N(R)--R'--O--P(O)(OM).sub.2 ] and a process for their preparation are disclosed in German No. 2,713,498. These compounds may be used as interface active agents.
The problem therefore arose of providing a foam inhibiting agent which would be effective at an extremely low concentration without affecting the general properties of the bath and without altering the mordanting action or deposition of metal on workpiece in galvanic processes.
To meet the technical requirements of a satisfactory foam inhibiting agent in highly acid solutions such as galvanic baths, the agent must meet the following criteria:
The foam inhibiting agent should generally act rapidly but its action should persist over a long period.
The method of adding the antifoamant to the acid bath should be very simple so that it can be widely and effectively used. This may be achieved, for example, by suitable formulation of the active ingredient in an aqueous or organic solution or suspension in water or organic liquids with or without the addition of co-surfactants. The solvent or dispersing agent used is preferably water although organic carrier substances could be used in the formulations since the additive is used in such very small doses in the bath that it should not interfere with the treatment of the workpiece.
In addition to the required surface activity, the agent must also be soluble or dispersible and it must be chemically extremely resistant to acid and oxidizing media even at elevated temperatures. It should be compatible with the salts present in the mordants or galvanic baths and should be effective even at the very lowest concentrations without interfering with the process and should be completely free of any adverse effects on the mordanting or metallizing process. In addition, any losses due to the additive being carried out with the bath should be easy to control and compensated by suitable subsequent additions. Since a co-surfactant is also used to lower the surface tension, the activity of the latter must, of course, also be unimpaired.